Field of the Invention
The present invention relates to a liquid seal type pressure sensor having a double diaphragm.
Description of the Background Art
A liquid pressure detecting pressure sensor is used for a consumer electrical appliance, an automobile air-conditioner etc. A pressure sensor (liquid seal type pressure sensor 10) in which a pressure transmission medium 2 (liquid) is tightly sealed inside a container 1 as shown in FIGS. 5A and 5B has been known as such a pressure sensor. FIG. 5A is a plan view of a diaphragm of the pressure sensor viewed from above. FIG. 5B is an explanatory sectional view taken along line 5A-5A in FIG. 5A. The liquid seal type pressure sensor 10 is typically used for higher pressure detection in comparison with a gas pressure detecting pressure sensor. The container 1 is formed to have a space in which the pressure transmission medium 2 (liquid) is tightly sealed between the metal diaphragm 3 and the pressure sensor unit 4. A concentric circular corrugated shape 3a is formed in the metal diaphragm 3 of the pressure sensor. Liquid such as oil 2a whose volume hardly changes due to pressure is preferably used as the pressure transmission medium 2 (liquid). As indicated by arrows in FIG. 6, a liquid pressure of an external liquid to be measured/detected is transmitted to the pressure sensor unit 4 through the metal diaphragm 3 having the corrugated shape 3a and the oil 2a tightly sealed in the sealed space. Thus, the liquid pressure of the external liquid can be detected. The liquid seal type pressure sensor 10 shown in FIGS. 5A and 5B and FIG. 6 includes a double diaphragm comprising the metal diaphragm 3 and a semiconductor silicon diaphragm (not shown in drawing) built in the pressure sensor unit 4. Accordingly, the liquid seal type pressure sensor 10 has an advantage that the pressure sensor unit 4 can be completely protected from the external liquid because the pressure sensor unit 4 is not directly contacting the external liquid.
A stainless steel material is mainly used as the material of the metal diaphragm 3 used in the liquid seal type pressure sensor 10. A stainless steel material processed to be 15 μm to 40 μm thick can be used. Other than stainless steel, non-ferrous metal such as phosphor bronze may be used. For example, silicone oil is preferably used as the pressure transmission medium 2 (oil 2a) for transmitting the pressure of the external liquid received by the metal diaphragm 3 to the pressure sensor unit 4 inside the container 1. The oil 2a is tightly sealed in the space between the metal diaphragm 3 and the pressure sensor unit 4. The oil 2a has a function to properly transmit the pressure of the external liquid received by the metal diaphragm 3 to the pressure sensor unit 4 when the oil 2a is at a constant temperature. However, the tightly sealed oil 2a changes in volume when the temperature changes. That is, when the oil 2a is heated, it expands from an original position indicated by a broken line as shown in FIG. 4. When the oil 2a is cooled, it contracts. The volume change caused by the temperature change functions as a pressure on an inner wall of the container 1 forming the tightly sealed space and also acts as a pressure on the pressure sensor unit 4 mounted in the same space, as indicated by arrows in FIG. 4. Accordingly, the volume change caused by the temperature change adversely affects temperature characteristics of the pressure sensor unit 4 conspicuously. The concentric circular corrugated shape 3a is formed in the metal diaphragm 3 of the pressure sensor. The object is to change the shape, size, etc. of the concentric circular corrugated shape 3a so as to reduce the spring modulus (rigidity) of the metal diaphragm 3 and increase the substantial area of the metal diaphragm in the same pressure receiving area to thereby suppress the increase of the internal pressure.
As to the liquid seal type pressure sensor having such a structure, a literature regarding an invention of a magnetoresistance transducer describes a temperature compensating diaphragm having a structure in which a plurality of diaphragms having different thermal expansion coefficients is laminated to one another to make temperature compensation based on a difference among the thermal expansion coefficients of the plurality of diaphragms (JP-UM-A-3-115831 (in Lines 10 to 13 in Claim (4) for Utility Model registration)).
In order to reduce deformation of the diaphragm caused by the temperature change, it has been described that stainless steels having different thermal expansion coefficients are used separately for an inner layer and an outer layer (JP-A-2009-186209 (Problem and Solution in Abstract)).
However, when the pressure receiving area of the metal diaphragm is reduced in response to a request for the miniaturization of a product etc., the spring constant for the same plate thickness or corrugated shape of the diaphragm as that in the conventional art is increased and the internal pressure is also increased greatly. Therefore, in order to further reduce the rigidity of the metal diaphragm, for example, reducing the plate thickness is anticipated. However, when the plate thickness is further reduced, secondary factors such as the processing limit of the corrugated shape, deformation caused by deterioration of the strength of the material, and difficulty of handling may newly occur.